In any language there is a mixing between ordinal numbers and fraction numbers, although they have nothing to do with each other. In general, in all languages, the first three numbers have their proper form. Beyond these numbers some languages uses the same form for the fraction and the ordinal numbers. (English is actually very restrictive concerning this issue. In roman languages for the first ten numbers there is no difference between the fraction number and the ordinal number.)
fraction numbers
ordinal numbers
a whole
first
a half
second
a third
third
a quarter
forth
a fifth
fifth
a sixth
sixth
a eighth
eigtht
a ninth
ninth
a tenth
tenth
Latin doesn't have fraction numbers and from the very beginning ordinal numbers were used as well as fraction numbers: primus, secundus, tertius, quartus, quintus, sextus, septimus, octavus, nonus, decimus. The only language known by the author that distinguishes completely between fraction numbers and ordinal numbers is german.
However beyond a certain size the ordinal numbers are so complicated, see 25.1. that it is is not possible to express fraction with ordinal numbres. 3/121 would be ~ três / centésimos vigésimos primeiros. Therefore in the course of history a new system was created. Fraction numbers beyond ten can be formed as well by adding -avo, although for some numbers the original ordinal number can still be used, at least in theory.
For the first ten fraction numbers the original latin ordinal numbers are used.
meio = halbe
1/2 = um meio (one half)
3 / 2 = três meios ( three half)
terço = drittel
1/3= um terço (one thirdl)
2 / 3 = dois terço (two third)
quarto = viertel
1/4 = um quarto (one quarter)
3 / 4 = três quartos ( three quarters)
quinto =fünftel
1/5= um quinto (one fifth)
4 / 5 = quatro quintos (four fifth)
sexto = sechstel
1/6 = um sexto (one sixth)
3 / 6 = três sextos (three sixth)
sétimo = siebtel
1/7 = um sétimo (one seventh)
5 / 7 = cinco sétimos (five seventh)
oitavo = achtel
1/8= um oitavo (one eighth)
7 / 8 = nove oitavos (seven eighthl)
nono = neuntel
1/9= um nono (one nineth)
8 / 9 = oito / nonos (eight nineth)
décimo = zehntel
1/10 = um decimo (one tenth)
4 / 10 = quatro décimos (four tenth)
For the fraction numbers beyond ten there are sometimes two forms.The tenth, the eleventh, the twelfth and the round fractions (tenth, twentyth, hundredth, four hundredth, thousandts, millionth etc.) have their own forms.
However even these numbers are monsters and difficult to remember because some of them doesn't steem from cardinal numbers and it is therefore to assume that the avo form is preferred.
fraction numbers
10
décimo
dez avos
11
undécimo
onze avos
12
duodécimo
doze avos
13
-
treze avos
14
-
quatorze avos
15
-
quinze avos
16
-
dezesseis avos
17
-
dezessete avos
18
-
dezoito avos
19
-
dezenove avos
20
vigésimo
vinte avos
21
-
vinte e um avos
30
trigésimo
trinta avos
40
quadragésimo
quarenta avos
50
quinquagésimo
cinquenta avos
60
sexagésimo
sessenta avos
70
septuagésimo
setenta avos
80
octogésimo
oitenta avos
90
nonagésimo
noventa avos
100
centésimo
cem avos
112
-
cento e doze avos
200
ducentésimo
duzentos avos
300
tricentésimo
trezentos avos
400
quadringentésimo
quatrocentos avos
500
quingentésimo
quinhentos avos
600
sexcentésimo
seiscentos avos
700
septingentésimo
setecentos avos
800
octingentésimo
oitocentos avos
900
nongentésimo
novecentos avos
1.000
milésimo
mil avos
1.100
-
mil e cem avos
1.500
-
mil e quinhentos avos
3597
-
três mil quinhentos e noventa e sete avos
10.000
décimo milésimo
dez mil avos
100.000
centésimo milésimo
cem mil avos
110.000
-
cento e dez mil avos
1.000.000
milionésimo
um milhão avos
(For those who are interested: In Spanish the form with -avos can be used as an ordinal number as well, but not in Portuguese. It is to assume that the Portuguese avoid construction like o ducentésimo quinquagésimo nono homem, the 259th man, by using a glossing-over, someting like o homem numero duzentos cinquenta e um. Concerning the origin of -avo there is a not very convincing theory. It is said that the term steems from ancient egipt, see FRAÇÕES?. Following this theory the ancient egypts had a measure to survey the land but not every land was exactly a multiple of this measure and therefore the used one eighth of this measure, for instance 4 + 2/8 in case that this was needed and eighth is octavus in latin and from this octavus only survived avus and avus becomes avo. The problem is, that the egypts never spoke latin, at most they spoke, after Alexander has conquered the country, greek or macedonian or whatever.)